1. Field of the Invention
The present invention is in the field of gamma ray testing of geological formations. In particular, the invention delineates boundaries of a formation from recorded spectra.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval. The natural gamma ray log is particularly useful since radioactivity in earth formations is primarily due to potassium in shales and clays, and thus serves as a lithology indicator.
In general, one such method and apparatus, when applied to the field of natural gamma ray well logging, has utilized channels of spectra including those of potassium-40, uranium and thorium. One such technique utilizes these natural gamma ray logs to determine characteristics of shale formations as described in U.S. Pat. No. 4,071,755, to Supernaw et al. Unfortunately, it can be difficult to differentiate such shale formations from other formations having relatively high concentrations of uranium, such as uranium-rich sands or formations exhibiting uranium oxide plating as a result of fluid movement.
U.S. Pat. No. 612,439 to Chace provides methods and apparatus for evaluating subsurface formations utilizing natural gamma ray spectral logging. A high-resolution, gamma ray spectrometer incorporated in a well logging instrument traverses a borehole, whereby natural gamma radiation strikes a scintillation crystal contained therein. The detected gamma rays striking the crystal cause the crystal to emit photons in the visible energy region, the intensity of which is proportional to the energy lost in the crystal by the incident gamma ray. Light energy from the crystal is optically coupled to a photomultiplier tube where the energy is convened to a proportional electrical pulse signal which is amplified and transmitted to processing circuitry.
Due to the finite size of the gamma-ray tool, the measured gamma-ray count is an indicator for the formation properties integrated only over some finite volume. In many circumstances, these integrated count values are adequate for the spatial resolution of lithology. In certain exploration areas the geologic strata of interest are less than 1 m thick and in some cases may be composed of interleaving thin beds on the order of a few centimeters thick. In the former situation the spatial averaging effects of the tool may be substantial; in the latter, bearing in mind the normal detector length of 0.15 m (0.5 ft), they may cause a distorted picture of the lithology. Deconvolution can be used to reduce the averaging effects of the tool, thereby increasing the vertical resolution and accuracy of the log. For noiseless data, this deconvolution can be done by employing a least-squares method, and the results are then accurate regardless of the spatial resolution demanded, as long as it is greater than or equal to the measurement interval. On the other hand, in the presence of noise, the accompanying degradation of deconvolved results is a sensitive function of the resolution. As the resolution interval decreases, the mathematical process of deconvolution tends to increase the noise in the results.
Sheng et al. (“Bayesian deconvolution of gamma-ray logs,” Geophysics. Vol. 52, No. 11 (November 1987): p. 1535-1546.) teach a Bayesian deconvolution method that takes into account the statistics of gamma-ray log measurements, is less sensitive to noise, and shows an improvement over the conventional least-squares deconvolution methods. Jacobson et al. (“Resolution Enhancement of Nuclear Measurements Through Deconvolution, ”SPWLA 31st Annual Logging Symposium, Jun. 24-27, 1990. ) have developed a modified deconvolution method that is less sensitive to noise than the conventional least-squares methods. The method used in Jacobson is based on a model in which the spatial response function of a gamma ray detector is approximated by the convolution of a rectangular box whose length is equal to the length of the detector with an exponential cusp function whose decay constant is related to the average gamma-ray attenuation length in common reservoir formations.
One problem of particular interest is simply that of identification of bed boundaries, particularly those of shale layers in a sand-shale sequence. Accurate resolution of thin beds is more important than deconvolving the entire log accurately. For such applications, a commonly used method has been to identify bed boundaries from inflection points on the log. This approach works reasonably well if the thickness of the shale bed is sufficiently large and the log reading is within 5% of the fully developed API value. The method however is inaccurate in thin beds. It would be desirable to have an improved method and apparatus for resolution of thin beds from natural gamma ray measurements. The present invention satisfies this need.